For producing complex components from aluminum plates by mechanical machining, for instance of tools for plastic injection molding, low-stress and high-strength input material is required.
The source of stresses in the input material is the internal stresses from the extrusion process, dictated by temperature gradients in casting, as well as internal stresses from the heat treatment; these are stresses caused by the quenching process. In the mechanical machining, stresses in the input material lead to an impairment of dimensional stability and thus to warping of the component. Typically, straightening is impossible because of close tolerances, and the workpieces have to be rejected.
For such usage objectives, the precipitation-hardenable wrought aluminum alloy EN AW-6082, an alloy of the AlMgSilMn type, has become especially well established. For producing plates, this material is cast into rectangular formats by extrusion and then, for molding the alloy elements that have been precipitated at the particle limits and to compensate for casting segregations (differences in concentration of alloy elements) is subjected to a first heat treatment (so-called homogenization). After that, a second heat treatment is effected for adjusting the mechanical properties. Between the first and second heat treatments, a reshaping step (such as rolling) may be effected.
The prior art here is the performance of full hardening, including solution annealing, ensuing quenching in cold water, and subsequent artificial aging. In the solution annealing, the hardness component magnesium silicide Mg2Si is dissolved by diffusion in the primary mixed crystal at temperatures of about 550° C. for 6 to 10 hours, depending on the format. With the quenching in cold water, which causes cooling to below 150° C. in less than 20 seconds, freezing of the state of equilibrium established at the solution annealing temperature occurs, which corresponds to a state of disequilibrium at room temperature. The ensuing artificial aging at temperatures of 150 to 200° C. for 8 to 15 seconds represents a targeted precipitation of the hardness component for adjusting the strength.
Aluminum bars treated in this way have very good mechanical properties, but because of the internal stresses that are present because of the quenching in cold water, they are unsuitable for use for mechanical machining. The aluminum bars are therefore subjected to a cold working in order to reduce the very great majority of the internal stresses from the quenching process. Following the heat treatment, the aluminum bars are stretched by means of hydraulic systems by from 1 to 5% of the original length.
Aluminum plates produced by this extensive method are distinguished by good mechanical strength, but are only in low-stress form, and warping during the mechanical machining can still occur.
The thermal mechanical strain on such aluminum plates, for instance in plastic injection molding, leads to a steady loss of strength and therefore leads to continuously increasing wear of the tool.
There is accordingly still a need for aluminum alloys from which low-stress, high-strength aluminum input material can be produced, such as a form of cast plates, which input material is suitable for mechanical further machining, for instance for producing base plates for plastic injection molding tools.